1. Field of the Invention
The present invention is related to the field of thermal transfer printing and, more particularly, to a method of controlling heater element activation in a thermal transfer printhead for improved print quality in cold ambient temperatures while retaining good printhead life.
2. Description of the Related Art
Thermal transfer printers operate through selective heating of a plurality of microscopic heater elements within the printhead. The heater elements, when activated, produce points of heat that correspond to each of the dots in an image line that is to be printed onto a medium. The heated heater elements, when thereafter coming into contact with the ink on an adjacent ribbon, heat and transfer the ink onto the medium in points or dots corresponding with the dots in the desired image line.
To effect this selective heating in order to produce a desired image, the printhead is provided with a ceramic wafer having a plurality of integrated circuits (ICs) and heater elements mounted thereon. The ICs are used to switch the heater elements on and off in response to data, loaded into the IC for each dot, indicating whether or not that dot is to be printed. The duration of the period for which the heater element is “on” is specified by the pulse width of the current flowing to the heater element, as controlled by appropriate printer control electronics.
There are three layers of logic in a driver IC. First, a shift register array accepts the data on heater element activation. Second, a latch array freezes the data in place. Finally, NAND gates are used to switch the current to the appropriate heater elements.
A conventional logic diagram for a driver IC that switches 64 dots is provided in FIG. 1. The 64 heaters labeled H1 to H64 are assumed to be connected to a common voltage source at their top and are switched to ground (GND) as shown by the row of NAND gates. The control signals enter the shift register of the IC at DATA IN (on the left) and exit at DATA OUT (on the right). The \LATCH signal initiates transfer of the data from the shift register to the latch array. The \STROBE and BEO pins are asserted during heater element activation as will be discussed more fully hereinafter.
Other ICs connected to the left and right of the IC shown make up the total printhead, with the DATA OUT shown in FIG. 1 becoming the DATA IN of a next IC to the right (not shown). The direction of data loading is left to right when viewing the printhead with the heater element line facing the viewer and the connector directed down or toward the viewer. The \STROBE signal is asserted by the printer control electronics for the length of time the current is to flow. The line over the \STROBE and \LATCH signals indicates that these signals are active low.
The sequence of operations for printing three lines is depicted in the diagram of FIG. 2, with the meaning of the data bits being “high” to print and “low” to not print.
The printer controller presents a data bit on the DATA IN pin and pulses the CLOCK pin. The printhead copies this data bit into the leftmost shift register on the rising clock pulse, with the bits in the other shift registers shifting to the right to make room. The controller repeats this step for a number of times equal to the number of heater elements on the printhead. Typically, a 53 mm printhead has 640 heater elements or pixels, while a 128 mm printhead has 1536 heater elements or pixels. This process is summarized in the drawings for the loading of a print line of six dots, progressing from before loading in FIG. 3A, to after the first clock pulse in FIG. 3B, and to the completion of five clock pulses, when the six data bits have been completely loaded, in FIG. 3C.
Once all of the data bits have been loaded, i.e., the shift registers contain data bits for each of the heater elements, the controller pulses the \LATCH pin low, which causes the printhead to copy all of the data bits to the latch registers, as shown in FIG. 4A. As can be seen in FIG. 2, once the data is latched, the controller can begin loading data for the next line, even though the first line is still printing.
Once the data is in the latch register, the controller asserts the \STROBE and BEO (block enable out) pins. Current will flow to all heater elements having a high data bit in their latch register for as long as \STROBE is low and BEO is high. Hence, as shown in FIG. 4B, the four heater elements with high data bits are switched on with current flowing for as long as \STROBE is held low.
The BEO pin is provided as a safety feature on most printheads to prevent heater element burnout in the event the controller leaves the current switched on for too long. Particularly during power on and power off, the printer controller electronics can be unstable and accidentally assert pins low or high. However, it is unlikely that both of these pins would be accidentally asserted at the same time.
In practical application, since the printhead only makes dots, higher level depictions of images such as characters, bar codes or pictures are first reduced to lines of dots by computer software or printer electronics so that each line of dots can be printed in sequence to produce the image. As each line of target dots is printed, some, all or none of the heater elements used in the previous line may be reused in the subsequent line. Heater elements that are not reused in two subsequent lines may yet be bounded on one or both sides by neighboring heater elements that were or are being used in the last or current line, respectively. This reuse and/or adjacent relationship of heated heater elements must be taken into account when printing an image in order to avoid reaching excessive operating temperatures within the heater elements involved. More particularly, heater elements that fire frequently grow hotter over the length of the page being printed, resulting in poor print quality from larger or darker printed dots and also reduced printhead life due to continued operation at excessive temperatures. By monitoring and/or tracking heater element activation, referred to in the industry and herein as history control, different pulse widths can be applied on a dot-by-dot basis, depending upon the heat that is assumed to be present in a target or neighboring dot from its having fired on previous print lines.
On a general basis, history control features have been incorporated within prior art driver ICs to reduce the pulse width to specific heater elements when it is known that those heater elements have retained heat from firing on the previous line. Driver ICs manufactured by KYOCERA, for example, utilize five control (CONT) signal inputs. In response to each of these five signal inputs, the printer controller provides pulses of progressively shorter duration. The choice of which signal input, and hence which pulse width, to direct to each dot is made based on that dot's immediate history and the immediate history of the dots adjacent thereto.
In accordance with the five signal inputs of Kyocera, each heater element that is going to fire can be unambiguously assigned to one of five scenarios which are illustrated in FIG. 5. As shown, when a heater element meets the criteria for a given scenario, e.g., CONT 3, that heater element cannot simultaneously be in another scenario, e.g., CONT 2.
The lowermost squares in the bottom line, referred to as line 3 as being the third line in the printing sequence shown, are black, indicating that these heaters have been selected, through appropriate data bit input, to fire on this print line. The two lines of squares above line 3 show the heater element activation which occurred in the previous two image lines, with the arrow showing the direction of the printhead movement relative to the paper.
The heater element in the scenario to which the CONT 1 signal is applied is relatively cold because it did not fire in the preceding two lines. The heater element to which the CONT 5 signal is applied, by contrast, is relatively hot, having been fired in both of the preceding two lines. In order to print the two dots in line 3 with the same optical density, the print controller must hold the CONT 5 signal low for a shorter period of time than the CONT 1 signal to reduce the pulse width to the “hot” heater element.
It takes some time for heat to diffuse from a heater element that fires to an adjacent heater element that did not fire. As a result, particularly at higher print speeds, it is more important to consider the contribution from adjacent dots on the prior line than those on the present line. Hence, the heating status of adjacent dots in line 3 are not shown, as they are not taken into consideration. A representative portion of a printed image, with the sequence of printed dots providing examples of these five scenarios, is shown in FIG. 6.
While the application of varying pulse widths to the heater elements through history control has been implemented, these implementations have relied upon very general power categorization. On a practical level, problems remain, particularly under certain environmental conditions. A primary environmental condition for which the known power level inputs have proven to be ineffective is that of extreme cold, i.e., temperature conditions of 5° C. or less. These conditions are commonly encountered in meat processing facilities in which meat is stored, processed, packaged and printed in refrigerated rooms for product preservation and consumer safety.
In such cold ambient temperatures, the cooling effect of the environment upon the printer heater elements results in inadequate ink transfer. The resulting print quality is poor, being too light and lacking sufficient uniformity and definition to ensure that the printed information is and remains legible as needed to inform the customer of the package content and handling instructions. Various solutions have been attempted but have not proven successful.
One solution is to increase the power directed to the heater elements in order to heat the ink sufficiently to obtain the needed print quality. This increase in power burns out the heater elements in a short time, however, resulting in permanently poor print quality under all environmental conditions. An example of poor print quality and the underlying pixel damage resulting in such quality is provided in FIGS. 7A and 7B, respectively. This permanent damage to the printhead also results in unacceptably short printhead life. For example, using conventional KYOCERA printheads at generally elevated power levels to compensate for temperature, printhead life was reduced from about 600,000 prints to 20,000 before the number of damaged heater elements prevents usable printhead performance. Failure was catastrophic, with dozens of heater elements or pixels burned out such that print performance capability was destroyed. Given that the average purchase price for a printhead is on the order of several hundred dollars, this reduction in life and associated increase in operating cost is entirely unacceptable, particularly in view of the large daily print volume required in a commercial meat packing plant.
Recognizing the high rate of printer pixel burnout resulting from generalized increases in the power applied to the heater elements, KYOCERA provides with their printheads recommended maximum limits on pulse width for each of the five CONT signals, as summarized in Table I.
TABLE IManufacturer Recommended Pulse Width LimitsspeedCont 1Conts 2&3Cont 4Cont 5(mm/sec)usecusecusecusec290????300180145112.58031017814311179320176141109.578330175140108.57734017313910775350172137.5105.573.536017113610472370170135102.57038016713199.56839016312796.566400158124946441015512292.56342015412191.56243015312090.561440????
These limits are provided with the conveyed understanding that operation above the indicated thresholds will result in premature printhead failure. However, when the printhead is operated within the manufacturer recommended limits under cold conditions, unacceptable print quality is obtained as has already been noted.
Therefore, a need exists for a thermal transfer printing method that operates effectively in cold ambient temperatures to produce good print quality without undue reduction in printhead life.